Permanent magnets are used in a variety of devices, including traction electric motors for hybrid and electric vehicles, as well as wind turbines, air conditioning units and other applications where combinations of small volumes and high power densities may be beneficial. Sintered neodymium-iron-boron (Nd—Fe—B) permanent magnets have very good magnetic properties at low temperatures. However, due to the low Curie temperature of the Nd2Fe14B phase in such magnets, the magnetic remanence and intrinsic coercivity decrease rapidly with increased temperature. There are two common approaches to improving thermal stability and magnetic properties at high temperatures. One is to raise the Curie temperature by adding Cobalt (Co), which is completely soluble in the Nd2Fe14B phase. However, the coercivity of Nd—Fe—B magnets with Co decreases, possibly because of the nucleation sites for reverse domains. The second approach is to add heavy rare earth (RE) elements such as dysprosium (Dy) or terbium (Tb), or both. It is known that the substitution of Dy for Nd or Fe in Nd—Fe—B magnets results in increases of the anisotropic field and the intrinsic coercivity and a decrease of the saturation magnetization. See, for example, C. S. Herget, Metal, Poed. Rep. V. 42, P. 438 (1987); W. Rodewald, J. Less-Common Met., V111, P 77 (1985); and D. Plusa, J. J. Wystocki, Less-Common Met. V. 133, P. 231 (1987). It is a common practice to add the heavy RE metals such as Dy or Tb into the mixed metals before melting and alloying.
However, Dy and Tb are very rare and expensive materials. Only a small fraction of the RE mines in the world contain heavy REs. The price of Dy has increased sharply in recent times. Tb, which is needed if higher magnetic properties are required than Dy can provide, is even more expensive than Dy. Furthermore, these metals may be difficult to work with in their relatively pure form, where for example pure Dy is too soft to form into a powder, and is also easily oxidized.
Typical magnets for traction electric motors in hybrid electric cars and trucks contain between about 6 and 10 weight percent Dy to meet the required magnetic properties, while other applications (such as the aforementioned wind turbines and air conditioners, as well as other vehicular configurations (such as motorcycles that may not have as high of an operating temperature environment as their car and truck counterparts) may have lower Dy needs. Assuming the weight of permanent magnet pieces is about 1-1.5 kg per electric traction motor, and a yield of the machined pieces of typically about 55-65 percent, 2-3 kg of permanent magnets per motor would be required. Moreover, because other industries compete with permanent magnets for limited Dy resources (thereby exacerbating already high costs associated with such materials), reducing the Dy usage in permanent magnets would have a very significant cost impact, as it would for Tb.
The microstructures of Nd—Fe—B sintered magnets have been extensively investigated in order to improve the magnetic properties of such magnets composed mainly of the hard-magnetic Nd2Fe14B phase and the nonmagnetic Nd-rich phase. The coercivity is known to be greatly influenced by the morphology of the boundary phases between Nd2Fe14B grains. The magnetic properties of the Nd—Fe—B sintered magnets are degraded when the magnet size is decreased because the machined surface causes nucleation of magnetic reversed domains Likewise, in their work entitled Improved Magnetic Properties of Small-Sized Magnets and Their Application for DC Brush-less Micro-Motors, Coll. Abstr. Magn. Soc. Jpn. 142 (2005), 25-30), Machida et al. found that the degraded coercivity of small-sized Nd—Fe—B sintered magnets can be improved by surface treating the formed magnet with Dy and Tb-metal vapor sorption so that there is a uniformly distributed coating of Dy or Tb on the outside of the formed magnet. While such approaches are helpful in improving the properties of magnets that have been treated with Dy or Tb, they do so at great expense by utilizing much of these precious materials.
Current embodiments provide advantages over sintering methods and provide for hot pressing and/or die-upset methods to increase Dy distribution along the grain boundary and to increase the non-uniformity of Dy distribution.